A B767-ER FLT IS MANDATED TO RETURN LAND AFTER TKOF WHEN IT IS REALIZED THAT THE NOSE GEAR DOWN LOCK PIN WAS STILL INSTALLED AND NOT REMOVED DURING PREFLT AT ZZZZ; FO.

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ALL THE PLTS ARRIVED AT THE ACFT AT THE SAME TIME. I CHKED THE LOGBOOK. ETOPS INSPECTION WAS WRITTEN IN THE BOOK; BUT NO WRITE-UP OF GEAR PINS BEING INSTALLED WAS IN THE BOOK. I WENT OUTSIDE FOR THE WALKAROUND OF THE ACFT. I STARTED ON THE L SIDE OF THE NOSE OF THE ACFT; LOOKING AT THE VARIOUS PROBES AND VANES ON THAT SIDE. I THEN STEPPED AWAY FROM THE ACFT TO LOOK AT THE CONDITION OF THE RADOME AND WINDSHIELD. CONTINUING BACK TOWARDS THE COPLT'S SIDE; I LOOKED AT THE OXYGEN BLOWOUT AND THE VARIOUS ON THAT SIDE. I THEN WALKED BEHIND THE NOSE GEAR AND LOOKED AT THE NOSE GEAR LINKAGE AND THE APU PANEL. I LOOKED UP; BUT SAW NO FLAGS. I CONTINUED THE WALKAROUND AS NORMAL. I REMEMBER LOOKING AT THE MAIN GEAR PINHOLES. THERE WERE NO PINS INSTALLED. IT WAS A NORMAL TAXI OUT AND TKOF. ON GEAR RETRACTION; THE GREEN NOSE GEAR LIGHT STAYED ILLUMINATED ALONG WITH AN AMBER GEAR DISAGREEMENT LIGHT. WE ALSO RECEIVED A GEAR DISAGREE EICAS MESSAGE. I GOT OUT THE CHKLIST; WORKING THROUGH IT WITH THE CAPT WHILE THE OTHER FO CONTINUED TO FLY THE ACFT. WE LOWERED THE GEAR AND HAD A NORMAL GEAR INDICATION OF 3 GREEN LIGHTS. WE RETURNED TO.ZZZZ; LNDG UNEVENTFULLY. WE WEIGHED ABOUT 375000 LBS. UPON ARR AT THE REMOTE PARKING SPOT; THE CONTRACT MECH (NON CARRIER PERSONNEL) CAME INTO THE COCKPIT AND INFORMED US THAT THE NOSE GEAR PIN WAS INSTALLED. HE DID NOT SAY WHY HE HAD INSTALLED ONLY THE NOSE GEAR PIN. THE CAPT ASKED WHY HE DIDN'T WRITE IT IN THE LOGBOOK (AS PER AIRLINE PROC) AND HE ANSWERED THAT HE THOUGHT ABOUT IT; BUT HAD BEEN DISTRACTED AND FORGOT. THE SUBSEQUENT TKOF AND LNDG AT XXX WAS UNEVENTFUL. FACTORS; PERCEPTIONS: IN MY 5 YRS OF FLYING THIS AIRPLANE INTERNATIONALLY; I DON'T EVER REMEMBER WALKING AROUND AN ACFT WITH GEAR PINS INSTALLED. MY ONLY EXPERIENCE HAS BEEN VIEWING THE WALKAROUND PHOTOS IN OUR RECURRENT TRAINING. I DID NOT WALK IN THE FRONT OF THE NOSE GEAR AND LOOK UPWARD. I ONLY LOOKED UP INTO THE NOSE GEAR WELL FROM BEHIND THE STRUT; WHICH HID THE HANGING FLAG ON A STILL DAY SUCH AS THIS. I DO NOT KNOW HOW FAR DOWN THE FLAG HANGS FROM THE NOSE GEAR PINHOLE. THE HOLE IS HIGH ENOUGH UP ON THE STRUT; THAT TO INSTALL THE PIN; A PERSON WOULD HAVE TO USE A LADDER OR STEP UP ON THE NOSE TIRE. THE MECH DID STATE THAT THE NORMALLY RED FLAG WAS DARKENED BY GREASE FROM THE WHEEL WELL. RECOMMENDATIONS: EXTEND THE LENGTH OF THE GEAR PIN FLAGS TO WELL BELOW THE NOSE GEAR DOOR. CHANGE THE COLOR OF THE FLAGS TO A BRIGHTER; MORE NOTICEABLE COLOR. REEMPHASIZE IN PLT TRAINING THAT THOUGH THE MAIN GEAR PINS ARE NOT INSTALLED; THE NOSE GEAR PIN MIGHT BE INSTALLED. REEMPHASIZE PROCS FOR GEAR PIN INSTALLATION TO CONTRACT PERSONNEL.